Function handle_declare_cursor 

async fn handle_declare_cursor(
    db: &LaminarDB,
    state: &ConnState,
    cursor_name: &str,
    subscribe: SubscribeStatement,
) -> PgWireResult<Response>

Open a SUBSCRIBE behind a cursor name. Rejects with 42P03 if the name is already in use on this connection (matches PG; user must CLOSE first).